This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Branching or scapose herbs, with alternate or basal leaves, and corymbose, paniculate or solitary, peduncled heads of both tubular and radiate (rarely all tubular) flowers. Involucre hemispheric, cylindric or campanulate, its bracts narrow, nearly equal, mostly imbricated in but 1 or 2 series. Receptacle nearly flat, usually naked. Ray-flowers, in our species, white, violet or purple, pistillate. Disk-flowers yellow, tubular, perfect, their corollas mostly 5-lobed. Anthers obtuse and entire at the base. Style-branches more or less flattened, their appendages short, mostly rounded or obtuse. Achenes flattened, usually 2-nerved. Pappus-bristles fragile, slender, scabrous or denticulate, in 1 series, or often an additional outer short series. [Greek, early-old, alluding to the early hoary pappus.]
A genus of 130 species or more, of wide geographic distribution, most abundant in the New World. In addition to the following, many others occur in the southern and western parts of North America. Type species: Erigeron acris L.
1. E. uniflorus.
†† Stem 4 -3 high, leafy, usually branched; involucre hirsute or glabrous. 1. Roots perennial, thick and woody. Heads 1'-2' broad; leaves lanceolate, ovate, oblong, or spatulate. Rays violet or purple; stem leaves ovate, lanceolate, or oblong. Stem leaves linear-lanceolate, the upper much smaller.
2. E. asper.
Stem leaves ovate-lanceolate, the upper little smaller.
3. E. subtrmervis.
Rays mostly white; stem leaves linear or linear-oblong.
4. E. caespitosus.
Heads 1/2' -1' broad; leaves linear.
Plants hirsute or canescent; pappus double; western species. Stem hirsute; achenes pubescent; flowers white.
5. E. pumilus.
Stem appressed-canescent; achenes glabrous; flowers purple or white.
6. E. canus.
Plant nearly glabrous; pappus simple; northern.
7. E. hyssopifolius.
2. Perennial by decumbent, rooting stems or stolons.
8. E. flagellaris.
3. Roots annual or biennial, fibrous; plants often perennial by offsets. Heads 1'-1 1/2' broad, few; stem simple; eastern.
9. E. pulchellus.
Heads 1/2' -1' broad, numerous; stem branched. Rays 100-150, narrow, mostly purple or violet.
Pappus simple; plant erect, corymbosely branched.
10. E. philadelphicus.
Pappus double; plant diffusely branched, western.
11. E. divergens.
Rays much less numerous, purplish or white.
Plants 6'-12' high, diffuse, western; pappus simple.
12. E. Bellidiastrum.
Plants 6'-30 high, erect, branched; pappus double. Pappus of the ray-flowers and disk-flowers unlike. Stem leaves lanceolate, nearly all sharply serrate.
13. E. annuus.
Stem leaves linear-lanceolate or oblong, nearly all entire.
14. E. ramosus.
Pappus of the ray-flowers and disk-flowers alike; plant hirsute.
15. E. tenuis.
††† Stem leafless or nearly so; heads 1/2' broad, corymbose.
16. E. vernus.
**Rays inconspicuous or short; a row of tubular pistillate flowers inside the row of rays.
17. E. acris.